Since the middle of the 20th Century, organic co-crystals have been of interest to a number of researchers. Saunder, D. H. Proc. R. Soc. London, Ser. A 1946, 188, 31-51; Vanniekerk, J. N., et al. Acta Crystallogr. 1948, 1, 44-44; Andrews, L. J. Chon. Rev. 1954, 54, 713-776; McConnell, H. J. Chem. Phys. 1954, 22, 760-761; McConnell, H. M., et al. Proc. Natl. Acad. Sci. U.S.A. 1965, 53, 46-50; and Desantis, F., et al. Nature 1961, 191, 900-901. Charge-transfer (CT) co-crystals, in particular, have been studied for their structural modularity and novel properties. Herbstein, F. H. Crystalline Molecular Complexes and Compounds: Structures and Principles, Oxford University Press: Oxford, N.Y., 2005; Klosterman, J. K., et al. Chem. Soc. Rev. 2009, 38, 1714-1725; Jerome, D., et al. Adv. Phys. 2002, 51, 293-479; Horiuchi, S., et al. J. Phys. Soc. Jpn. 2006, 75, 051016; Saito, G., et al. Bull. Chem. Soc. Jpn. 2007, 80, 1-137. They are modular, inexpensive, and solution-processable materials that can be designed to exhibit properties such as ferroelectricity, conductance, magnetism, and optical nonlinearity. Although the properties of these crystals are well understood, there has been very little research aimed at incorporating them into organic electronic devices.
The lattice is composed of an electron deficient molecule, the acceptor (A), and an electron-rich constituent, the donor (D). When the donor and acceptor are complexed, an electron wave oscillates between them, i.e., the CT. In the most basic model, the CT interaction can be viewed as a charge donation from the donor HOMO to the acceptor LUMO. Torrance, J. B., et al. Phys. Rev. Lett. 1981, 46, 253-257. More comprehensive research on the ground state of DA co-crystals reveals, however, that the CT interaction actually varies significantly in terms of its structure and complexity. Murata, T., et al. J. Am. Chem. Soc. 2007, 129, 10837-10846; Saito, G., et al. Philos. Trans. R. Soc. London, Ser. A 2008, 366, 139-150. For convenience, CT is typically categorized by the parameter ionicity (p) that represents the degree of electron donation (0≤p≤1) between the donor and the acceptor
      (                  D        ⁢                  ⟶                      e            -                          ⁢        A            =                        D                      +            P                          ⁢                  A                      -            P                                )    .
Electron donor-acceptor ordered networks are good candidates for organic ferroelectrics because of the possible long range orientation of charge transfer dipoles. The canonical electron donor-acceptor (DA) systems, the mixed stack tetrathiafulvalene (TTF) with halogenated quinones, like TTF⋅chloranil (TTF⋅QCl4) and TTF⋅bromanil (TTF⋅QBr4), have been investigated by X-ray crystallography, vibrational spectroscopy, and electrical measurements. Horiuchi, S., et al. Science 2003, 299, 229-232 (2003); Horiuchi, S., et al. Nature Mater. 7, 357-366 (2008); Collet, E. et al. Science 300, 612-615 (2003); Kagawa, F., et al. Nat Phys 6, 169-172 (2010); Torrance, J. B. et al. Phys. Rev. Lett. 47, 1747-1750 (1981); Girlando, A., et al. J. Chem. Phys. 79, 1075-1085 (1983); Okamoto, H. et al. Phys. Rev. B 43, 8224-8232 (1991); Soos, Z. G. Chem. Phys. Lett. 440, 87-91 (2007); and Kagawa, F. et al. Phys. Rev. Lett. 104, 227602-227606 (2010). The TTF⋅QCl4 complex undergoes a ferroelectric phase transition, associated with a discontinuous jump in ionicity (ρ) at the Curie temperature (Tc=81 K), and dimerization into DA pairs (D0 A0 D0 A0 □ Dδ+Aδ− Dδ+Aδ−) breaking centro-symmetry. Categorizing this critical point as a ferroelectric transition was first postulated in 1991 when an anomalous dielectric spike was also observed at Tc, for TTF⋅QCl4. The TTF⋅QBr4 crystal, already ionic (ρ>0.5) at room temperature, also dimerises into DA pairs at 53° K as result of a spin-Peierls instability. Girlando, A., et al. Solid State Commun. 54, 753-759 (1985). Even with a ferroelectric ground state, however, measuring reversible polarization under an electric field has only been shown in TTF⋅QBr4.
Conventional organic CT crystals can be co-crystallized into two different packing arrangements, segregated stacks and mixed stacks. Anderson, P. W., et al. Solid State Commun. 1973, 13, 595-598; Iwasa, Y., et al. Phys. Rev. B: Condens. Matter 1990, 42, 2374-2377; Kuwatagonokami, M., et al. Nature 1994, 367, 47-48; and Hamilton, D. G., et al. Aust. J. Chem. 1997, 50, 439-445. In segregated stacks, the donor and acceptor pack edge-to-edge in separate columns (DDD, AAA), while in crystals with a mixed stack motif, the donor and acceptor occupy alternating positions (DADADA) along the CT axis. These two packing arrangements have considerably different physical properties. Co-crystals with segregated stacks typically exhibit metallic conductivity since the overlapping n orbitals between stacks of open shell donors and acceptors merge into conduction bands. Jerome, D. Chem. Rev. 2004, 104, 5565-5591. A mixed stack system is primarily known for polar phase transitions with changes in temperature, variations in pressure and optical excitation. Bruinsma, R., et al. Phys. Rev. B: Condens. Matter 1983, 27, 456-466; Masino, M., et al. Phys. Chern. Chern. Phys. 2001, 3, 1904-1910; Iwasa, Y., et al. Synth. Met. 1991, 42, 1827-1830; Tokura, Y., et al. Solid State Cornmun. 1986, 57, 607-610; Girlando, A., et al. Solid State Commun. 1986, 57, 891-896; and Koshihara, S., et al. Phys. Rev. B: Condens. Matter, 1990, 42, 6853-6856. Other exotic physical phenomena, like nonlinear electronic transport, magnetic ordering, and optical nonlinearity, have been identified in mixed stack crystals as well. Ferraris, L., et al. J Am. Chern. Soc. 1973, 95, 948-949; Samoc, M., et al. J. Chem. Phys. 1983, 78, 1924-1930; Massa, D., et al. Mol. Cryst. Liq. Cryst. Sci. 1989, 175, 93-117; Kondo, R., et al. Chem. Lett. 1999, 333-334; Kondo, R., et al. Synth. Met. 2001, 120, 995-996; Mitani, T, et al. Phys. Rev. Lett. 1984, 53, 842-845; Tokura, Y, et al. Phys. Rev. B: Condens. Matter 1988, 38, 2215-2218; Iwasa, Y, et al. Phys. Rev. B: Condens. Matter 1989, 39, 10441-10444; Hughes, R C., et al. J Chem. Phys. 1968, 48, 1066-1076; Huizinga, S., et al. Phys. Rev. B: Condens. Matter 1979, 19, 4723-4732; Hasegawa, T, et al. Solid State Commun. 1997, 103, 489-493; Kagawa, F., et al. Nature Phys. 2010, 6, 169-172; Rao, S. M., et al. J Appl. Phys. 1991, 70, 6674-6678; Ezaki, H., et al. Solid State Commun. 1993, 88, 211-216; Mazumdar, S., et al. Chern. Phys. 1996, 104, 9283-9291; Wong, M. S., et al. Adv. Mater. 1997, 9, 554-557; Zyss, J., et al. Chern. Mater. 2003, 15, 3063-3073.
Research that relies on organic co-crystals presents numerous challenges. Most notably, it can be difficult to grow high quality crystals that are large enough for experiments in integrated systems and devices. Being able to produce these materials quickly and reproducibly would facilitate their use in basic research and also in applications. It is therefore desirable to provide a self-assembly platform which amplifies the molecular recognition of donors and acceptors and produces co-crystals at ambient conditions.